1. Field of the Invention
The invention relates to a liquid crystal display device, and more particularly, to a lamp electrode printed circuit board of a backlight unit for a liquid crystal display device and a backlight unit including the same.
2. Discussion of the Related Art
According to the rapid development in information technology, flat panel display (FPD) devices, such as liquid crystal display (LCD) devices, plasma display panels (PDPs), electroluminescent display (ELD) devices and field emission display (FED) devices, have been developed and have replaced cathode ray tubes (CRTs).
Among these devices, liquid crystal display (LCD) devices are most widely used for monitors of notebook computers, monitors of personal computers and televisions due to excellent moving images and high contrast ratio.
Liquid crystal display (LCD) devices use the optical anisotropy and polarization properties of liquid crystal molecules of a liquid crystal layer to produce an image. The liquid crystal molecules have long and thin shapes, and because of the optical anisotropy property, the polarization of light varies with the alignment direction of the liquid crystal molecules. The alignment direction of the liquid crystal molecules can be controlled by varying the intensity of an electric field applied to the liquid crystal layer. Accordingly, an LCD device includes two substrates spaced apart and facing each other and a liquid crystal layer interposed between the two substrates. Each of the two substrates includes an electrode on a surface facing the other of the two substrates. A voltage is applied to each electrode to induce an electric field between the electrodes and the alignment of the liquid crystal molecules as well as the transmittance of light through the liquid crystal layer is controlled by varying the intensity of the electric field.
The LCD devices require an additional light source because the LCD devices are not self-luminescent. Therefore, a backlight unit is disposed at a rear side of a liquid crystal (LC) panel and emits light into the LC panel, whereby discernible images can be displayed.
Backlight units are classified into an edge type and a direct type according to the position of a light source with respect to a display panel. In edge-type backlit units, one or a pair of lamps are disposed at one side or each of two sides of a light guide panel of a backlight unit. In direct-type backlight units, a plurality of lamps are disposed directly under a display panel.
The edge-type backlight units can be easily fabricated, but the edge-type backlight units are disadvantageous to a large size display device. On the other hand, the direct-type backlight units are advantageous to a large size display device because of uniformity of light.
Lamps are classified by cold cathode fluorescent lamps (CCFLs), each of which has electrodes inside of a tube, external electrode fluorescent lamps (EEFLs), each of which has electrodes outside of a tube, or electrodeless fluorescent lamps (EFLs), which do not have electrodes.
FIG. 1 is an exploded perspective view of schematically illustrating a liquid crystal display (LCD) module including a direct-type backlight unit according to the related art. In FIG. 1, a related art LCD module includes a liquid crystal panel 10, a back light unit 20, a support main 30, a top cover 40 and a cover bottom 50. The liquid crystal panel 10 substantially displays images. A gate printed circuit board 18 and a source printed circuit board 19 are attached at respective sides of the liquid crystal panel 10 adjacent to each other via flexible circuit boards 16.
The backlight unit 20 is disposed under the liquid crystal panel 10. Side surfaces of the liquid crystal panel 10 and the backlight unit 20 are surrounded by the support main 30 having a rectangular frame shape. The top cover 40 covers edges of a front surface of the liquid crystal panel 10, and the cover bottom 50 covers a rear surface of the backlight unit 20. The top cover 40 and the cover bottom 50 are combined with the support main 30 to thereby constitute one-united body.
The backlight unit 20 includes a reflection sheet 22, a plurality of lamps 24 arranged over the reflection sheet 22, and a plurality of optical sheets 26 over the lamps 24. Both ends of each lamp 24 are fixed by respective side supports (not shown), which are combined with the cover bottom 50.
The lamps 24 are used as a light source of the backlight unit 20 and include external electrode fluorescent lamps, each of which has electrodes outside of a glass tube. The lamps 24 are spaced apart from and arranged in parallel with each other over the reflection sheet 22. External electrodes of each lamp 24 are connected to common electrodes 23, which may be referred to as a lamp electrode, and thus a voltage is simultaneously applied to the external electrodes of each lamp 24.
The liquid crystal display device module is referred to as a so-called rear front mounting structure, wherein the module is assembled and combined by screws 60 passing through four corners of the cover bottom 50 and then inserted into the support main 30. In the structure, heads of the screws 60 are disposed at a rear surface of the cover bottom 50, and the exterior of the module is tidy.
However, since the liquid crystal display device module of the rear front mounting structure is assembled by only the screws 60 passing through the four corners, this may weaken a fixed force and decrease reliability in keeping a shape of the module. Accordingly, to increase the fixed force, screws may be further used along four edges in addition to the four corners of the liquid crystal display device module. But, in regions A corresponding to two opposite edges of the liquid crystal display device module, where the common electrodes 23 for the lamps 24 are formed, it is difficult that the screws are used owing to the common electrodes 23.
FIG. 2 is an enlarged view of the region A of FIG. 1. In FIG. 2, each lamp 24 has electrodes 28 at both ends, and common electrodes 23 are disposed at both ends of each lamp 24. The common electrodes 23 are adjacent to inner surfaces of two edges of the cover bottom 50 facing each other. Therefore, even though screws are used along the two edges of the cover bottom 50, the screws may not pass through the common electrodes 23 formed of a metallic material, whereby two edges of the liquid crystal display device module cannot be combined.
In addition, an inverter and driving elements for applying voltages to the common electrode 23 are formed on a backlight printed circuit board (not shown), which is disposed at a rear surface of the cover bottom 50 of FIG. 1. Therefore, a lamp wire (not shown) for electrically connecting the common electrode 23 and the backlight printed circuit board (not shown) should be stably and efficiently connected to common electrode 23.